Risk assessment provides an objective, technical evaluation of the likelihood of unacceptable impacts to human health and the environment. Considerations of risk are also used to decide which problems need to be dealt with most urgently, This process of decision making and its consequent actions are called risk management.
This process of risk-based decision making provides a clear framework for those involved in contaminated land decisions to consider the acceptability of risks posed by contaminants at a site, either before or after treatment, and how any necessary risk reduction can be achieved efficiently and cost effectively.
Risk assessment is based on considering linkages between sources, pathways and receptors. These “pollutant linkages” are useful not just in assessing risks from contamination, but also understanding them is the key to effective remediation. There are three basic ways in which pollutant linkages can be broken, as illustrated in Figure 1:
· source reduction, (for example removal of a leaking tank and its surrounding contaminated soil)
· pathway management (for example using a barrier to restrict the flow of contaminated groundwater), and
· modifying exposure of the receptor (for example by choosing a future land use where opportunities for exposure are reduced).
Figure 1 Breaking Pollutant Linkages
1.1 Reducing or Modifying Sources
Contamination may exist as “point sources”, localised zones of high contaminant concentrations, such as those associated with long-term or major releases from a defined location
Typical point sources may include substances in their original position (e.g., buried tanks, tar pits, lagoons and waste deposits) and contaminants in bulk quantities from the original release, such as NAPLs (non-aqueous phase liquids) in soil fissures or floating on the water table. Diffuse sources of contamination are where contaminated material has been spread over wide area, for example by atmospheric fallout from a smelter, or activities such as sewage sludge spreading.
The expression “source term” is often used to describe both the original source and any zone of highly contaminated materials that it may have spread to. The “traditional” approach to treating point sources has been removal followed by disposal. Increasingly excavation may be followed by ex situ treatment, either to effect a complete treatment of the excavated material (for example biodegradation of contaminants) or to separate out relatively clean materials and so reduce the volume of material requiring off site disposal. From a waste management perspective, treatment is a form of waste minimisation since it reduces the amount of waste leaving the contaminated site. Source term removal by excavation may simply be impossible for diffuse contamination problems, because of the sheer volume of material requiring excavation, which also limits the usefulness of ex situ treatment approaches.
There are alternatives to excavation for removing the source term. A number of technologies aim to remove contamination by biological or chemical degradation, or to physically remove them. Physical removal techniques include the use of pumping or suction or venting to move fluids, which may be enhanced by the use of heat, surfactants or artificially created fracture zones in the subsurface
1.2 Pathway Management
Pathway management encompasses a variety of approaches that are used to disrupt the contaminant transport pathway from source to receptor. These approaches include passive barriers, such as impermeable bentonite walls driven through an aquifer. These reduce the flow of groundwater, and so the rate at which the contaminant is transferred to the receptor. Active pathway management techniques include pump and treat based processes, in situ treatment and permeable reactive barriers (PRBs).
Pump and treat describes techniques where groundwater is removed from an aquifer via wells and treated above ground, followed either by re-infiltration or discharge to surface water (depending on permits). In situ remediation describes interventions where some kind of degradation or removal of contaminants is initiated within the pathway, so reducing the level of contamination reaching the receptor. PRBs describe a particular configuration of in situ treatment, which at a conceptual level introduces a barrier to the contamination, but not to the fluid (carrier) that was carrying the contamination. The contamination is treated within or in the vicinity of the PRB. The aim of PRBs is to direct the flow of a carrier into a relatively small volume, which can be more carefully managed as a treatment system.
Considering the capacity of naturally occurring processes to mitigate the impacts of contaminants has become an important part of pathway management.
1.3 Modifying Exposure of the Receptor
Risk management at the point of the receptor involves altering the behaviour or presence of receptors in some way so as to reduce the risks to which they are subjected. The most common example of this is recognising some limitations to the use of land to avoid particular pathways. In effect, "fitness for purpose" recognises that for a particular use of land remediation need only be to the extent that no risks are posed to receptors of concern. The absence of risks may be because the pollutant linkage is broken because a particular receptor will not be present (for example the land will be used as a car park, not for growing food), or because its planned use means that a pathway will not be present, for example concrete hardstanding in a car park preventing direct contact with contaminated soil. As a short term action removal of receptors is possible but is unlikely to be a sustainable solution, as such evacuation in most cases would be likely to lead to more dereliction. If the risks concern the extraction of drinking water one may move the extraction well or choose to treat the extracted water, although this may not be a ‘preferred option’.
Risk management is therefore a process of deciding how pollutant linkages might be most effectively and efficiently broken, and then undertaking the actions which have been agreed as necessary.
At the heart of any risk management work must be the derivation of a site conceptual model (SCM). The SCM integrates what is already known about a site, identifies what still needs to be discovered, and how site information should be used. The SCM sets out the critical linkages between sources-pathways and receptors for a particular land contamination problem.. Strategic approaches to risk management must also take into account the limitations of risk management as a technique. Linking risk management and sustainable development, for example in the “Risk Based Land Management” approach is increasingly accepted as an optimal approach to the problems of contaminated land management.
There are a number of factors that need to be considered in selecting an effective remediation solution to a contaminated land problem. These include the reasons for the remediation work and any constraints on it, risk management, technical suitability and feasibility, stakeholders’ views, cost/benefit ratio and wider environmental, social and economic impacts (i.e. sustainable development). It is also important to consider the manner in which a decision is reached. This should be a balanced and systematic process founded on the principles of transparency and inclusive decision-making.
Verification and validation have slightly different meanings. Verification is a series of activities and measurements that enable the effectiveness of a particular operation on site to be assessed, and hopefully confirmed. Validation is a series of activities and measurements that have been carried out to ensure a particular technique is fit for its general purpose. Verification and validation typically encompass a range of information, matching actual to predicted findings: the remediation effectiveness; cost; use of resources (personnel, reagents, energy etc.); generation of wastes and emissions; impact on soil/water properties (e.g. pH, redox).
Typically a remediation technique will need to have some validation for a potential user to take it seriously. For any given remediation project the site owner and regulator will want to have the effectiveness of the remediation verified.
The relationship of risk management related to the current or future use of a site includes an implicit assumption that land use will not change to one that is more sensitive to any hazards left in place after remediation work. If a change to a more sensitive land use takes place, the processes of risk assessment and risk management must be repeated. Multi-functionality is where suspected contaminated land was to be cleaned sufficiently to allow any end use. However, by the end of the 1990s it had become clear that no country could justify the technical and economic resources needed. Multi-functionality was also questioned from the perspective of use of resources versus sustainable development. It may not be an optimal use of scarce resources to treat land to a degree that is likely never to be required, or at least not required for several decades. An important consequence of the use of risk management related to site use is that adequate records must be kept for the future.
Extracted from Nathanail & Bardos, (2004) “Reclamation of Contaminated Land” ISBN 0471985600